In ancient foundry practice, molds were prepared from green pressed sand. The molds consisted of a cope (upper) and a drag (lower) portion. The cope and drag were placed together and the central cavity between them, which bore the geometry of the desired part, was filled with molten metal. When the metal had solidified, the cope and drag were removed, leaving a shape that required relatively little machining, known as "a near net shape."
In modern practices, the sand has been replaced with pressed, disposable "Bakelite" molds, various "Styrofoam" cavity formers, wax impressions, ceramic dipped wax and fired molds, vacuum investment, and other types of molds. While these molds offer various advantages over sand, it is still very expensive to create a mold. As a result, it is economically necessary to cast a large number of identical parts, so that each part need only bear a small portion of the cost of the mold. Casting prototypes, for example, where only one or two castings is needed, is very expensive. Moreover, exotic metals, such as titaniun, require expensive rammed graphite molds. The necessity for using a mold in order to cast a shape also increases the length of time needed to design and cast new shapes, because of the time required to design and manufacture new molds. While shapes can also be formed by forcing molten metal through dies so that the metal emerges cooled and solidified from the other end of the die, dies limit the casting to a single cross-sectional shape and are also expensive and time consuming to construct. Shapes can also be formed by the hot isostatic pressing of powders, but that also requires shaped containers, using expensive high pressure procedures.